1. Field of the Invention
The present invention relates to a method of treating a zinc oxide film that is formed by electrochemical deposition, a method of manufacturing a photovoltaic device utilizing the same, and a zinc oxide film treatment apparatus.
2. Related Background Art
Photovoltaic devices including hydrogenated amorphous silicon, hydrogenated amorphous silicon germanium, hydrogenated amorphous silicon carbide, microcrystalline silicon, or polycrystalline silicon, are conventionally provided with back side reflection layers in order to improve light-collection efficiency in the long-wavelength regions. Such reflection layers desirably show effective reflection characteristics at wavelengths which are close to band edges of semiconductor materials and at which absorption becomes small, i.e., wavelengths of 800 nm to 1,200. Those that can fulfill such conditions are reflection layers formed of a metal such as gold, silver, copper, or aluminum, or alloys thereof. Also, the following attempt, which is known as the light entrapment experiment, has been done: an irregular transparent electroconductive layer, which is optically transparent, is provided within a predetermined wavelength region so that reflected light can effectively be utilized to improve short-circuit current density Jsc. Further, the transparent conductive layer also prevents characteristics from deteriorating owing to shunt pass. In most general cases, those layers are deposited by a process such as vacuum evaporation or sputtering and show an improvement in short-circuit current density.
As an example thereof, in “Light Entrapment Effect in a-SiGe Solar Cells on 29p-MF-22 Stainless Steel Substrates” (Autumn, 1990), The 51st Applied Physics Society Scientific Lecture Meeting, Lecture Drafts p.747, “P-IA-15a-SiC/a-Si/a-SiGe Multi-Bandgap Stacked Solar Cells with Bandgap Profiling”, Sannomiya et al., Technical Digest of The International PVSEC-5, Kyoto, Japan, p.381, 1990, reflectance and texture structure are studied on a reflection layer constituted of silver atoms. In this example, it is reported that the reflection layer is deposited in double layer of silver by changing the substrate temperature, to form effective irregularities, and an increase in short-circuit current in virtue of light entrapment effect has been achieved by combination with the zinc oxide layer.
Japanese Patent No. 3273294 discloses that a zinc oxide film formed from a zinc oxide film manufacturing electrolytic solution, which is composed of an aqueous solution containing 0.001 mol/l to 0.t mol/l of zinc ions and 0.001 mol/l to 0.5 mol/l of nitrate ions, is uniform in thickness and composition and has excellent optical transparency.
Japanese Patent Application Laid-Open No. H10-140373 discloses a zinc oxide thin film manufacturing method which includes a step of forming a first zinc oxide thin film on a substrate by sputtering and a step of immersing the substrate in an aqueous solution that contains at least nitrate ions, zinc ions, and carbohydrate to form a second zinc oxide thin film on the first zinc oxide thin film by causing a current to flow between the substrate and an electrode that is immersed in the solution. According to this manufacturing method, a zinc oxide thin film that shows excellent adhesion to a substrate can be formed at low costs without causing abnormal growth of the film.
Japanese Patent Application Laid-open No. 2001-152390 discloses that a zinc oxide film formed from a zinc oxide film manufacturing electrolytic solution that is composed of an aqueous solution has high electric resistance owing to water absorbed in the zinc oxide film but is reduced in resistance when dried by heating. Also disclosed in Japanese Patent Application Laid-Open No. 2002-237606 is that a solar cell obtained by forming a semiconductor layer on a zinc oxide film whose water content is 7.5×10−3 mol/cm3 or lower is improved in photoelectric conversion efficiency.
However, when a photovoltaic device is manufactured by forming a semiconductor layer on a zinc oxide thin film that is placed on a metal layer, a back side reflection layer of high reflectance is indispensable for further improvement in photoelectric conversion efficiency. To improve the photoelectric conversion efficiency, in particular, short circuit current, of a photovoltaic device having as a back side reflection layer a zinc oxide film that is formed by depositing on a silver, aluminum, copper, or like other metal thin film on a substrate, the back side metal layer should reflect an increased amount of light toward a semiconductor layer of the photovoltaic device by reducing light absorption of the zinc oxide film which is formed by deposition on the metal layer. If the electric resistance of the zinc oxide film is optimized at the same time, the short circuit current value of the photovoltaic device and the curved factor can be improved even more. Japanese Patent Application Laid-Open No. 2002-237606 points out that reduction in water content of a zinc oxide film slightly lowers the reflectance of the film while the electric resistance of the film is reduced.
It is also preferable for improvement in photoelectric conversion efficiency, short circuit current, in particular, of a photovoltaic device to elongate an optical path by surface irregularities on a back side reflection layer. One method to achieve this is to thicken a zinc oxide film of the photovoltaic device. In this case, however, the electric resistance of the zinc oxide film could be raised owing to the water content of the zinc oxide film (on the premise that a thicker zinc oxide film contains more water) and a contamination source could be created by steam expelled from a substrate when the substrate is heated in a later step (semiconductor deposition step).
In addition, its manufacture process needs to shorten the treatment time of a heat treatment step, uniformize characteristics throughout a large-area substrate region, and reduce the cost of a treatment apparatus.